A thin film formed so that it is epitaxially grown on a crystalline substrate has its properties influenced by its crystalline perfectness. For example, in the preparation of an oxide superconductor thin film such as of (Ba, Sr) CuO type which is epitaxially grown on a substrate to form a laminated superconductor, its superconductive transition temperature and superconductive critical magnetic field are influenced by its crystalline perfectness such as its crystal defect density and crystallographic orientation. Also, an epitaxial BaTiO3 thin film used as a memory element in a semiconductor integrated circuit has its capacity value largely varied by its crystallographic orientation.
So far, in order to obtain a high quality thin film that is satisfactory in crystalline perfectness, a substrate has been used having an in-plane lattice constant that is close to the in-plane lattice constant of the thin film. If there does not exist any substrate having an in-plane lattice constant close to that of a thin film, a material is chosen having an in-plane lattice constant intermediate between those of the substrate and the thin film and is layered on the substrate as a buffer layer on which the thin film may be grown.
It is, however, only rare that a substrate is available that agrees in in-plane lattice constant with the thin film, and if such a substrate is available, it often is extremely brittle or of high cost. In the use of a buffer layer, too, it rarely is the case that a substrate is available which is fully congruous in in-plane lattice constant.
Thus, in the past, since it has not been possible to grow a thin film on a substrate that fully agrees or is congruent in in-plane lattice constant therewith, it has been likely the case that a thin film has dislocations introduced therein due to its lattice mismatch with the substrate; hence a thin film results that is highly dense with crystal defects, imperfect in crystalline orientation, and thus poor in quality and properties, problems met by the prior art.